The invention relates to a method and device for modifying a zone in successive images forming a sequence, representing an evolutionary scene, delivered by at least one camera.
The term "image sequence" must be interpreted in a wide sense as designating not only the image signal (analog or digital) produced in real time by a camera or a video controller but also a sequence recorded on an inscribable filing medium (digital or analogue), on a photographic, magnetic or optical medium, in a form allowing subsequent display, delivery or broadcasting at a rate equivalent to the real time rate.
More precisely, the invention relates to a method and device for replacing an image fraction corresponding to a precise region or to an object of the scene which will be named hereafter "target area", with a representation of another precise region, or another object, which is real or not, which will be named hereafter "pattern".
The fields of application of the invention are extremely various. The following may be mentioned, non limitatively
advertising, the method making it possible to add or replace advertising panels during transmissions or retransmissions of sporting events or others;
production of audio-visual works, particularly fictional works, the method then making it possible to replace the scenery during shooting of production with another scenery;
simulation, in which the method makes it possible to insert the representation of a real object in an environment formed by a synthetic image.
Numerous methods are already known which can be used for modifying a target area in an image. The target area may for example be identified by a particular colour, which is preferably unused in the rest of the image to avoid any imbiguity. But this method does not take into account the image acquisition conditions, for example camera movements. The pattern cannot consequently follow the evolution of the scene satisfactorily.
In image synthesis a pattern may also be generated which is inlaid in a target area, but on condition, that position, focal distance and focussing do not change.
These methods, as well as the use of superimposed shots for producing motion pictures associating real scenes and animated cartoon characters, have limitations which exclude use thereof or reveal the special effects whenever the image acquisition conditions vary significantly in time.
Last, there is known (EP-A-0 360 576) a method enabling to generate a composite sequence from two sequences of video pictures. An operator manually indicates at least all reference points which represent the corners of a polygon and a transformation circuit modifies the addresses of the pixels in the polygon for enabling combination.
That method requires a manual operation for each picture.